SEG‐U NLP Field Trip Report June 2018 Copper, Gold and Silver Deposits in the II Region, Atacama Desert, Northern Chile June 11 – 16, 2018 Field Trip Report Field trip participants with the Llullaillaco volcano behind. Left to right: Morena Pagola, Agustina Esnal, Priscilla Myburgh, Julieta Palomeque, Camila Ferreyra, Florencia Pereyra, Diego Palma, Agustín Ulloa, Alejandro Toloy, Andrea Muñoz and Camila Riffo (Prof. Chong´s students), Patricio Arias (Prof. Chong’s long time field partner), Erick Montenegro (Prof. Chong´s student), Prof. Chong and Facundo De Martino 1 SEG‐U NLP Field Trip Report June 2018 Trip Leader: Dr. Guillermo Chong Trip Participants: De Martino, Facundo Julián Esnal, Agustina Ferreyra, Camila Myburgh, Priscilla Solange Pagola, Morena Lucía Sonia Palma, Diego Sebastián Palomeque, Julieta Pereyra, Florencia Toloy, Alejandro Daniel Ulloa, Agustín The Student chapter is deeply thankful to Prof. Guillermo Chong from UCN for organizing the visits to the mines and for all other support during the field trip. We also like to thank Guanaco Mine, Mining company Mantos de la Luna, Mining company El Peñón and Mining company Antucoya for providing meal and lodging, and the Society of Economic Geologist for providing financial support. Introduction From June 11th to the 16th the UNLP-SEG Student Chapter visited the II Region in northern Chile, where important gold&silver and copper mines are located. The schedule, detailed below, included Antucoya Mine, El Peñón, Mina Guanaco, Mina Mantos de La Luna, and a geologic tour led by Prof. Guillermo Chong, which included the Atacama Salt Flats. The first is a Cu porphyry mine, Peñón is a low sulphidation epithermal Au-Ag deposit whereas Guanaco is a high sulphidation Au- Cu deposit. Mantos de la Luna corresponds to a stratabound Cu deposit and the Atacama Salt Flat which is exploited for Li and nitrate. Field Trip Schedule • First Day (June 11 th): Arrival at Antucoya mine (Cu Porphyry) where we were given the mine induction talk, geological-mining model introduction and later visit to one open pit and then a dynamic leaching pile. • Second day (June 12 th): Visit to El Peñón underground mine (Au&Ag LS epithermal), after induction and geological-mining model introduction. Afterwards, further talks about the project´s history were given in the complex´s panoramic viewpoint. • Third day (June 13 th): Guanaco Mine. After geological-mining model introduction we were taken to a panoramic viewpoint and then to Dumbo Pit, now inactive, to visualize the mineralizing ledges. • Fourth day (June 14th): In the morning we arrived in the town of Tocopilla where Mantos de La Luna mining company gave us the mine induction and geological introduction at the company´s administrative facilities. Afterwards we were taken to the camp and mine where more specifics talks were given about the mine, and then we visited open pit exploitation. 2 SEG‐U NLP Field Trip Report June 2018 • Fifth day (June 15 th): geological tour starting in Antofagasta City, passing through Mina La Escondida (Cu porphyry), Atacama Salt Flat, short visit to San Pedro de Atacama town and Salt Range, return to Antofagasta passing nearby the city of Calama and Chuquicamata Mine and a short stop at La Escondida iron sulphates deposit. Regional Geology The II Region of Chilean topography is dominated by three main ranges, Coastal, Domeyko and Main Range (Picture 1). Most of the nearby basins in northern Chile are formed as the result of late Tertiary to Holocene faulting, and fault movement has affected the configuration of salt flats and the morphology of their crusts. In general, the major subregions containing salt flats are closely related to major tectonic elements of northern Chile. The Coastal Range and Central Valley subregion are within a zone of intensive block faulting associated with uplifting of the Coastal Range. The Atacama Basin and nearby subregion lies to the east and partly within a fault zone trending north-northeast, associated with the uplifted tectonic block of Domeyko Range. The Andean Highlands subregion is in the western Andes, an area of intensive volcanism and faulting during the late Tertiary and Quaternary period. Northern Chile is underlain chiefly by marine sedimentary rocks, volcanic rocks, and plutonic rocks of Jurassic and Cretaceous periods, which are mostly widespread in the Coastal Range, and by rhyolitic to basaltic volcanic rocks of Tertiary and Quaternary periods, which are most widespread in the Andean Highlands. Extensive areas are covered with thick alluvium and lacustrine sediments of late Tertiary and Quaternary periods. 3 SEG‐U NLP Field Trip Report June 2018 Figure 1. Regional Geology of II Region, Chile. ANTUCOYA The mine is located in the Antofagasta Region (Picture 2), also known as the II Region, 125 km northeast of Antofagasta City, with an altitude of 1700masl. The main characteristics of this mine are the use of sea water in the leaching process and the low copper grade within the mineralized rocks. 4 SEG‐U NLP Field Trip Report June 2018 Picture 2. Location of Antucoya Mine. Antucoya’s annual production consists of 85,000 tons of fine copper through open pit exploitation and dynamic piles leaching since 2015, which final product is the copper cathode. The actual measured resources exceed 1,000 million tons with 0.33% copper. The ore deposit is linked to cretaceous porphyries (140-142 Ma) (Maksaev et al., 2006) hosted in Jurassic rocks from La Negra Formation. Such ore is emplaced also following two structural trains consisting in two main fault zones. The first one is N- S oriented corresponding to Buey Muerto, and the second one is Antucoya, NW-SE oriented. Both define a horseshoe shaped zone (Picture 3) and both are linked to the Antofagasta Fault Zone (ZFA)(Dallmeyer et al., 1996; Scheuber and Andriessen 1990). W E Antucoy Buey ZFA a Muerto Picture 3. Aerial photograph of Antucoya and Buey Muerto pits and fault zones. 5 SEG‐U NLP Field Trip Report June 2018 Antucoya´s ore minerals consist on Pyrite, Chalcopyrite, Chrysocolla, Brochantite, and some Cu oxides, with potassic alteration development in the nucleus and phyllic alteration towards the edges. The porphyry’s root was located 700 meters below drill collar, being interpreted as the system’s deep zone. On the other hand, Buey Muerto zone presents pyrite as it’s characteristic mineral, and quartz-sericite alteration, with acid pH hydrothermal fluids. In this sector the system’s root hasn’t been found so it has been interpreted as a shallower zone than Antucoya’s. The differences between both sectors seem to rely on a block tectonic with a relative vertical displacement between them (Picture 4). 40 0 m ? ? Picture 4. Geological block diagram. Left: Antucoya block. Right: Buey Muerto block. The deposit exploitation consists in 9 phases of open pit, from which the first three have been already developed (Picture 5). 6 SEG‐U NLP Field Trip Report June 2018 Picture 5. Photography in the Phase 3 pit. The rock processing begins with the grinding and gathering of the debris in dynamic piles (Picture 6), where sea water with sulphuric acid (8 grams per liter) are used in the leaching process through aspersion and dripping, completing a 77 days cycle (8 days of humidification, 65 of aspersion/dripping and 4 of resting). The obtained solution by the leaching process is treated in an extraction plant through organic thinners and electrowinning in order to obtain the final product: high purity (99.99%) copper cathodes. Picture 6. Panoramic photograph of dynamic piles with its irrigation system. 7 SEG‐U NLP Field Trip Report June 2018 EL PEÑÓN El Peñón district of northern Chile is located near the eastern margin of the Central Depression, about 165 km southeast of Antofagasta (Picture 7). The deposit is considered to be a typical low sulphidation epithermal Au-Ag deposit. Picture 7. Location of El Peñón Mine. The deposit is located within a north-south trending belt of epithermal deposits of Paleocene age, within the Central Depression of the Atacama Desert. The deposit is hosted by late Upper Paleocene to Lower Eocene rhyolitic, overlying Upper Cretaceous and Paleocene andesite and dacite, as we could see in the drill cores (Picture 8). The distribution of Cretaceous and Eocene volcanic rocks is controlled by graben structures bounded by north-northeast trending faults, Dominador fault and La Mula fault. These are steeply dipping regional-scale structures with displacements of hundreds of metres. The principal direction of late dikes and main faults are parallel to the bounding faults. The faults dip steeply eastward on the east side of the property and westward on the west side, implying a horst/graben extensional structure. 8 SEG‐U NLP Field Trip Report June 2018 Picture 8. Drill Core observation at El Peñón. The gold-silver mineralization is restricted to banded quartz-adularia veins, hydrothermal breccias and minor quarts stockwork. It comprises disseminations of electrum, native gold and silver, acanthite, silver sulphosalts and halides, plus accessory pyrite occurring with quartz, adularia, carbonates, and clays. Geochronology on supergene alunite and Mn-oxide indicates that oxidation of the El Peñón deposit occurred between 27 and 14 Ma. The deposits were also estimated to contain approximately 17.5 million tonnes of inferred resources at an average grade of approximately 1.7 g/t Au and 60 g/t Ag, which is approximately 960,000 ounces of gold, and 33.5 million ounces of silver. 9 SEG‐U NLP Field Trip Report June 2018 The major grades are located in Bonanza, with 20 g/Tn of Au and 100 g/Tn of Ag. The underground mining (Picture 9) method used at El Peñón is the Bench and Fill Method. Picture 9.